1. Field of the Invention
The present invention relates to a novel extrusion die for the production of thermoplastic film web material, and more particularly, relates to an annular extrusion die for extruding a tubular thermoplastic film, which incorporates a novel filtering arrangement in the die body for the formulation of a more consistently or uniformly distributed polymer melt conveyed to the die orifice of the extrusion die, to there impart improved properties to the extruded thermoplastic film material. Moreover, the invention relates to a method of producing an improved extruded thermoplastic film web material through the utilization of the inventive extrusion die arrangement incorporating the novel filter for the polymer melt.
In essence, pursuant to the concept of imparting improved properties to a thermoplastic film web material, the invention provides for the incorporation of a central cavity forming a filtering chamber within the body of an annular extrusion die, which is filled with a particulate inert material of either uniform granular size or of random sizes, which will force the polymer melt being conveyed therethrough towards the die orifice to randomly flow through the interstices which are present between the particles of the granular material, and thereby cause the polymer to uniformly fill the cavity. Such a utilization of an inert particulate material forming a filter, especially a particulate material possessing a granular consistency, in a central flow chamber arranged in the die body upstream of the discharge orifice of the annular extrusion die, enables the following properties to be imparted to the polymer melt prior to its extrusion into a tubular thermoplastic film web material:
1. a high degree of filtration;
2. homogenation of high molecular weight gels;
3. a high shear field which erases the previous shear history of the polymer melt;
4. causing the melt to change direction, cross-sectional area, or to be distributed throughout the chamber without the creation of differential shears in the polymer melt.
In effect, the inventive annular extrusion die which is employed in producing tubular thermoplastic film webs imparts improved shear histories to the polymer melt, distributes the molten melt about the circumference of the die orifice, and develops a mass flow which facilitates the formation of a good caliper for the final thermoplastic film.
2. Discussion of the Prior Art
In the technology relating to the extrusion of tubular thermoplastic film webs through continuous molding processes, various types of annular extrusion dies are widely employed, of which the majority consist of annular extrusion dies which are of the side fed, spreader plate, spider and spiral die type, as is well known in the art. Each of these different extrusion die designs possesses both positive and negative features imparting different characteristics to the extrudate. For instance, the side fed die provides varying residence times to the polymer melt and produces a major weld in the extruded tubular film; the spider die and the spreader plate die each create differential shears and a large number of, although somewhat smaller, weld lines in the film; whereas, finally, the spiral die is extremely expensive to manufacture, and also produces differential shears in the film, which although less pronounced than that produced by the other types of dies, still presents a significant problem relative to the quality of the extruded tubular film material.
Although different types of extrusion devices are currently known in the art which provide for improvements in the homogenizing mixing and dispersion of polymer melts for high viscosity films, whereby such devices incorporate suitable filter structure, none of these devices are essentially analogous with that of the present invention, nor are they capable of imparting the properties to the polymer melt which is necessary for the production of a blow molded thermoplastic film material possessing superior characteristics.
Orso, et al. U.S. Pat. No. 3,888,963 discloses an extrusion apparatus for plastically moldable ceramic batch materials, which provides for in-line homogenization of the extrudable materials. This homogenization is effected by passing the materials through a plurality of screens and permitting the material to be admixed within a central chamber within the extrusion apparatus. However, in this instance, there is no provision for any granulate material filling the chamber in order to implement the random distribution of a polymer melt within an extrusion die to impart the improved properties thereto as contemplated by the invention.
Kalman U.S. Pat. No. 3,940,335 describes a filtering process and apparatus in which a substance, such as a viscous polymer material, is passed through a filter band which positioned to extend over a flow passageway, with the filter being passed through inlet and outlet ports to prevent leakage. However, there is no disclosure that any granulate material be incorporated within an extrusion die chamber as contemplated by the present invention so as to impart improved properties to a polymer melt prior to extrusion into a thermoplastic film material.
Finally, Russell, Jr. U.S. Pat. No. 4,260,350 discloses a filter for high viscosity liquids, in which a granulate material is interposed in a hollow cylindrical configuration between filter screens so as to provide for some dispersion of a high viscosity molten polymer supplied in a spiral or spinerette-type flow along a cylindrical path encompassing a hollow central space and to improve the mixing and filtering properties thereof. However, there is no disclosure of such a granular material filter being positioned as the filler in a central chamber in the body of an extrusion die having a single annular opening to produce a more uniform and random distribution of the polymer melt which will impart the properties to a tubular thermoplastic film web as is contemplated by the present invention.